Use OS-included Python 2.7 on macOS

[LRFLEW]

This fixes building duktape*, as well as other ports that depend on python2 (I haven't done a full scan for what else would be included). I didn't find a Github issue for this, but it does relate to PR #10030.

This updates vcpkg_find_acquire_program(PYTHON2) to change PROGNAME from python2 to python on macOS. Apple includes Python 2.7 under this name, but not as python2, in all (supported) OS releases. It is deprecated, but it is included in 11.0 Big Sur from what I've read, and because Python 2 was removed from Homebrew, this will allow it to work for now at least.

* duktape still has a minor issue caused by easy_install needing to be executed as root, but installing pyyaml ahead of time allows the build to succeed, even with easy_install failing.

[ras0219]

LGTM; could you also consider adding a message to duktape about it's pyyaml dependency? Something like

if(CMAKE_HOST_SYSTEM_NAME STREQUAL "Darwin")
  message(STATUS "Duktape requires pyyaml to build -- this is available with `easy_install pyyaml`")
endif()

or somesuch

[LRFLEW]

I was planning on making an issue for the pyyaml duktape issue once this was merged. (I was trying to keep this change in the tooling separate from the changes in the port). There seems to be some complexity to it, because the fix could be changing easy_install pyyaml to either sudo easy_install pyyaml or easy_install --user pyyaml. There's also the fact that the easy_install command requires having run brew install libyaml ahead of time (from what I can tell at least).

[strega-nil]

@LRFLEW thanks!